The invention relates to a composition and method for reducing lost circulation when aqueous or oil based drilling fluids are used. More particularly, the method involves injecting a first mixture and then a second mixture into the well bore and lost circulation zone. The first mixture is acidic and contains a polymer that absorbs water under alkaline conditions. The second mixture is alkaline and contains a base fluid and a hardening material, wherein the hardening material hardens as the base fluid seeps into the lost circulation zone and increases the pH in situ to the point that the polymer absorbs water and expands. The expanded polymer and hardened material plug fissures and thief zones.
Drilling fluids, or drilling muds as they are sometimes called, are generally slurries of clay solids or polymers used in the drilling of wells in the earth for the purpose of recovering hydrocarbons and other fluid materials. Drilling fluids have a number of functions, the most important of which are: lubricating the drilling tool and drill pipe which carries the tool, removing formation cuttings from the well, counterbalancing formation pressures to prevent the inflow of gas, oil or water from permeable rocks which may be encountered at various levels as drilling continues, and holding the cuttings in suspension in the event of a shutdown in the drilling and pumping of the drilling mud.
For a drilling fluid to perform these functions and allow drilling to continue, the drilling fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts, or in some cases, practically all of the drilling fluid may be lost to the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole.
Most wells are drilled with the intent of forming a filter cake of varying thickness on the sides of the borehole. The primary purpose of the filter cake is to reduce the large losses of drilling fluid to the surrounding formation. Unfortunately, formation conditions are frequently encountered which may result in unacceptable losses of drilling fluid to the surrounding formation despite the type of drilling fluid employed and filter cake created.
A variety of different substances may be pumped down well bores in attempts to reduce the large losses of drilling fluid to fractures and the like in the surrounding formation. Different forms of cellulose are often employed. Some substances that have been pumped into well bores to control lost circulation include almond hulls, walnut hulls, bagasse, dried tumbleweed, paper, coarse and fine mica, and even pieces of rubber tires.
Another process that is employed to close off large lost circulation problems is referred to in the art as gunk squeeze. In the gunk squeeze process, a quantity of a powdered bentonite is mixed in diesel oil and pumped down the well bore. Water injection follows the bentonite and diesel oil. If mixed well, the water and bentonite harden to form a gunky semi-solid mess, which will reduce lost circulation. Problems frequently occur in trying to adequately mix the bentonite and water in the well. The bentonite must also be kept dry until it reaches the desired point in the well. This method is described in U.S. Pat. No. 3,062,823.
Many of the methods devised to control lost circulation involve the use of water expandable clay such as bentonite, which may be mixed with another ingredient to form a viscous paste or cement. U.S. Pat. No. 2,890,169 discloses a lost circulation fluid made by forming a slurry of bentonite and cement in oil. The slurry is mixed with a surfactant and water to form a composition comprising a water-in-oil emulsion having bentonite and cement dispersed in the continuous oil phase. As this composition is pumped down the well bore, the oil expands and flocculates the bentonite, which, under the right conditions, forms a filter cake on the well bore surface in the lost circulation area. Hopefully, the filter cake will break the emulsion causing the emulsified water to react with the cement to form a solid coating on the filter cake. But, such a complex process can easily go wrong.
U.S. Pat. No. 3,448,800 discuses another lost circulation method wherein a water-soluble polymer is slurried in a nonaqueous medium and injected into a well. An aqueous slurry of a mineral material such as barite, cement or plaster of Paris is subsequently injected into the well to mix with the first slurry to form a cement-like plug in the well bore.
U.S. Pat. No. 4,261,422 describes the use of expandable clay such as bentonite or montmorillonite, which is dispersed in a liquid hydrocarbon for injection into the well. After injection, the bentonite or montmorillonite will expand upon contact with water in the formation, thus, it is hoped that the expanding clay will close off water producing intervals but not harm oil-producing intervals.
A similar method is disclosed in U.S. Pat. No. 3,078,920, which uses a solution of polymerized methacrylate dissolved in a nonaqueous solvent such as acetic acid, acetic anhydride, propionic acid and liquid aliphatic ketones such as acetone and methyl ethyl ketone. The methacrylate will expand upon contact with formation water in the water producing intervals of the well.
It has also been proposed to mix bentonite with water in the presence of a water-soluble polymer that will flocculate and congeal the clay to form a much stronger and stiffer cement-like plug than will form if bentonite is mixed with water. U.S. Pat. No. 3,909,421 discloses such a fluid made by blending a dry powdered polyacrylamide with bentonite followed by mixing the powdered blend with water. U.S. Pat. No. 4,128,528 claims a powdered bentonite/polyacrylamide thickening composition prepared by mixing a water-in-oil emulsion with bentonite to form a powdered composition that rapidly becomes a viscous stiff material when mixed with water. U.S. Pat. Nos. 4,503,170; 4,475,594; 4,445,576; 4,442,241; and 4,391,925 teach the use of a water expandable clay dispersed in the oily phase of a water-in-oil emulsion containing a surfactant to stabilize the emulsion and a polymer dispersed in the aqueous phase. When the emulsion is sheared, it breaks and a bentonite paste is formed which hardens into a cement-like plug. The patent discloses the use of such polymers as polyacrylamide, polyethylene oxide and copolymers of acrylamide and acrylic or methacrylic acid.
U.S. Pat. No. 4,124,748 discloses a cross-linked copolymer of a vinyl ester and an ethylenically unsaturated carboxylic acid or derivative thereof that can absorb 200-800% of its weight in water and expand substantially in volume when doing so. Another highly water absorbent, expanding copolymer is described in U.S. Pat. No. 4,230,040. The described compound is derived by polymerizing acrylic acid and/or methacrylic acid in the presence of polyvinyl alcohol, followed by neutralization and heat treatment.
U.S. Pat. No. 4,635,726 discloses the use of superabsorbent polymers dispersed or, suspended in a liquid hydrocarbon for injection into a well and placed at the location where lost circulation is occurring. After placement and upon mixing with water, the superabsorbent polymer expands thus decreasing the loss of fluid from the well bore.
U.S. Pat. Nos. 4,664,816; 4,836,940; 5,034,139; and 5,086,841 disclose various methods of utilizing water absorbent polymers to decrease the loss of circulation fluid in a well bore.